SSL means the information being submitted to your cart is encrypted and harder to read as it travels across the web. Not storing credit card information and using authorized.net gives people less to break into. Webscans with McAfee identify obvious problems. But it's just the surface... being truly secure goes beyond the logos used to instill confidence.
Personally I'd drop the Authorize.net logo as no consumer knows them and pick up a visa / master card logo in their place, or leave the spot empty. Two decently known marks from an SSL and Security firm will represent you well. If you want to add more, consider social validation with BizRate, Facebook Likes [the count implies usage] or something similar.
The goal is to show consumers you take things seriously... but not to go so far as to have your footer look like a list of icons.